ether as a nucleophile. Using aminothioether instead of aminal as a precursor of N-acyliminium was of high importance to the success of this otherwise disfavored 5-endo-Trig cyclization. A Hf(OTf)4-catalyzed (0.1 equiv) transformation of aminal to aminothioether was uncovered in the course of this study, allowing the conversion of tricyclic aminal 24 to aminothioether 25 to be realized in high yield